let x = `<head>啊啊啊啊`;
console.log(x.replace(/<head>/, 'abc')); //abc啊啊啊啊
console.log(x.replace(/<head>/, '$&abc')); //<head>abc啊啊啊啊

//$&能在被匹配到到的字符串的后面追加内容
